The present invention relates to a driver circuit for a light emitting element, and more particularly to a driver circuit for driving a light emitting element with a signal generated by superimposing, over the pulse string of a digital main signal, an analog sub-information signal whose frequency is lower than the bit rate of the main signal.
In a prior art optical modulating circuit of this kind, there is used a light emitting element driver circuit which involves two transistors having their emitters connected to each other (see the Japanese Laid-Open Patent Disclosure No. 57-171854). Out of the two transistors of this light emitting element driver circuit, to the collector of one (a first transistor) is connected the light emitting element, so that when the first transistor is ON, the other (a second transistor) is OFF, and vice versa The digital main signal is supplied to the base of either transistor, the first transistor for instance, and the first transistor is turned ON and OFF according to the high ("1") or low ("0") level, respectively, of the digital main signal to turn the light emitting element on and off. The lower frequency analog sub-information signal which is supplied to the common emitter terminal of the first and second transistors, controls the current flowing to the light emitting element according to the amplitude of the analog sub-information signal, and thereby modulates the optical intensity of the light emitting element.
However, if the supply of the digital main signal is prevented by circuit trouble or any other cause, the first transistor will remain turned OFF, and no more driving current will be supplied to the light emitting element. Thus the prior art optical modulating circuit has the disadvantage of being unable to transmit the analog sub-information signal and, consequently, to transmit monitoring information or the like contained in the analog sub-information signal when the digital main signal is intercepted.
Moreover, since the prior art optical modulating circuit has no feedback circuit to prevent the amplitude fluctuation of the analog sub-information signal supplied to the driver circuit, it has another disadvantage that the analog sub-information signal component of the optical signal is destabilized by the amplitude fluctuation of the analog sub-information signal.